Singleton code snippet and code template : https://blog.francois.raminosona.com/singleton-snippet-template

singleton.snippet

<?xml version="1.0" encoding="utf-8"?>
<CodeSnippets xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
  <CodeSnippet Format="1.0.0">
    <Header>
      <Keywords>
        <Keyword>Singleton</Keyword>
      </Keywords>
      <Title>singleton</Title>
      <Author>Francois Raminosona</Author>
      <Description>Adds a singleton pattern to the current class</Description>
      <Shortcut>singleton</Shortcut>
      <SnippetTypes>
        <SnippetType>Expansion</SnippetType>
      </SnippetTypes>
    </Header>
    <Snippet>
      <Code Language="CSharp">
        <![CDATA[#region Singleton

        private static $className$ _instance;

        public static $className$ Current => _instance ??= new $className$();

        #endregion

        private $className$()
        {
            // TODO : Delete other constructors
            // TODO : Make $className$ sealed
        }]]>
      </Code>
      <Declarations>
        <Literal Editable="false">
          <ID>className</ID>
          <Function>ClassName()</Function>
          <Default>className</Default>
        </Literal>
      </Declarations>
    </Snippet>
  </CodeSnippet>
</CodeSnippets>

singleton.template.xml

<?xml version="1.0" encoding="utf-8"?>
<CodeTemplates version="3.0">
  <CodeTemplate version="2.0">
    <Header>
      <_Group>C#</_Group>
      <Version />
      <MimeType>text/x-csharp</MimeType>
      <Shortcut>singleton</Shortcut>
      <_Description>Adds a singleton design pattern</_Description>
      <TemplateType>Unknown</TemplateType>
    </Header>
    <Variables>
      <Variable name="className1" isEditable="false">
        <Function>GetCurrentClassName()</Function>
      </Variable>
      <Variable name="className2" isEditable="false">
        <Function>GetCurrentClassName()</Function>
      </Variable>
      <Variable name="className3" isEditable="false">
        <Function>GetCurrentClassName()</Function>
      </Variable>
      <Variable name="className4" isEditable="false">
        <Function>GetCurrentClassName()</Function>
      </Variable>
      <Variable name="className5" isEditable="false">
        <Function>GetCurrentClassName()</Function>
      </Variable>
    </Variables>
    <Code><![CDATA[#region Singleton

        private static $className1$ _instance;

        public static $className2$ Current => _instance ??= new $className3$();

        #endregion

        private $className4$()
        {
            // TODO : Delete other constructors
            // TODO : Make $className5$ sealed
        }]]></Code>
  </CodeTemplate>
</CodeTemplates>